1. Field of the Invention
The present invention relates to a photoresist coating method and apparatus for coating a photoresist solution on a substrate such as a semiconductor wafer.
2. Discussion of the Background
A photolithographic process step of a semiconductor device manufacturing process includes, for example, a resist coating step for forming a photo-resist film on a semiconductor wafer (hereinafter, referred to as "wafer") and a developing step for developing a resist-coated wafer exposed to light.
To coat a resist solution uniformly over the wafer surface, a spin coating method is widely used.
In the spin coating method, a semiconductor wafer is vacuum-fixed on a spin chuck. While the semiconductor wafer is being rotated together with the spin chuck, a resist solution is supplied to a center of the surface of the semiconductor wafer from a resist nozzle which is positioned above the semiconductor wafer. The resist solution thus supplied is spread toward the circumference of the semiconductor wafer with the help of centrifugal force. Although the supply of the resist solution is terminated thereafter, the semiconductor substrate is continuously rotated in the same manner except that the rotational speed is gradually decreased. In this way, the resist solution spread over the semiconductor wafer surface is shaken off and dried.
Recently, it has been strongly desired to reduce resist consumption in order to reduce a manufacturing cost. To meet the requirement, the spin coating is generally performed by supplying the resist solution to a center of the semiconductor wafer while the wafer is being rotated at a high speed.
However, there is a problem when the semiconductor wafer is rotated at a high speed as mentioned above. That is, defects in coating, namely, ripple marks, are likely to generate on the resist film formed on the semiconductor wafer surface. The ripple marks are presumably ascribed to ripples formed of the resist solution which is supplied from the resist nozzle immediately before termination of the supply. More specifically, the resist solution supplied immediately before the termination of the resist solution supply spreads over the wafer outward in the form of ripple by the rotation of the wafer and then dry up before uniformly spread over the wafer surface. This is because the higher speed the wafer is rotated, the faster the resist solution gets dry. In this mechanism, ripple marks are considered to be left on the surface. On the other hand, with the tendency of reducing the thickness of the resist film, the resist material low in viscosity has been preferably used. With this tendency, the surface tension of the resist film decreases. As a result, the ripples are likely to appear in the resist film formed of the low-viscosity material, compared to the conventionally-used resist film.
To avoid the formation of the ripples, it is necessary to control the resist nozzle accurately to set at the center of the wafer surface. If so, the resist solution supplied can be spread concentrically to form a concentric circular ripple. It is also necessary to control the rotational speed of the semiconductor wafer to set the same at a speed which permits the resist solution to dry with such a timing that the concentric circular ripple of the resist solution reaches and passes the edge of the semiconductor wafer. However, it is very difficult to control the resist nozzle to position the same accurately and to control the rotational speed. Therefore, it has not been attained to completely avoid the formation of the ripples.
It is also difficult to control the thickness of the coated film when the amount of the resist solution supplied to each wafer is reduced. It is therefore difficult to obtain a film having a uniform thickness. Particularly in the case where the resist solution is supplied dropwise in an amount of 1.0 mL or less per wafer, the resist solution is dried before the supplied resist solution is spread outward and reach the edge of the wafer. It follows that the film thickness of the outer peripheral portion becomes thinner than that of the center portion. It is thus difficult to control the thickness of the coated film and therefore difficult to form a film in a predetermined uniform thickness.
As a method of controlling the thickness of the coated film, conventionally known are (a) a method of decreasing a speed of a resist solution spurted from a nozzle, and (b) a method of increasing the wafer rotation number when the resist solution is spurted out. However, the method (a) has a problem in that the resist solution is spurted unstably. In the method (b), the rotational velocity of the wafer cannot be increased without limitation. The increase is limited by the performance intrinsic to the coating apparatus. There is another problem in the method (b) in that the resist solution dries up too fast to control the film thickness.